Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus-v5

[David Daney]

On 01/31/2012 04:17 PM, Venkatesh Pallipadi wrote:
> Kernel's notion of possible cpus (from include/linux/cpumask.h)
>   *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
>
>   *  The cpu_possible_mask is fixed at boot time, as the set of CPU id's
>   *  that it is possible might ever be plugged in at anytime during the
>   *  life of that system boot.
>
>   #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
>
> and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
> and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
>
> i.e, We needlessly go through this mask based calculation everytime
> num_possible_cpus() is called.
>
> The problem is there with cpu_online_mask() as well, which is fixed value at
> boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
> in HOTPLUG case.
>
> Though most of the callers of these two routines are init time (with few
> exceptions of runtime calls), it is cleaner to use variables
> and not go through this repeated mask based calculation.
>
> Signed-off-by: Venkatesh Pallipadi<venki@xxxxxxxxxx>
> Acked-by: Srivatsa S. Bhat<srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> Acked-by: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>

How is it that this patch got merged to linux-next before all the 
cleanup patches for nr_online_cpus?

From the looks of your follow-on patches it would seem that all MIPS, 
hexagon, and um are now broken.

I know for a fact that MIPS doesn't boot because of this.

David Daney


> ---
>   include/linux/cpumask.h |    9 +++++++--
>   kernel/cpu.c            |   13 +++++++++++++
>   2 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 4f7a632..930e255 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -27,6 +27,9 @@ typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t;
>   extern int nr_cpu_ids;
>   #endif
>
> +extern unsigned int nr_online_cpus;
> +extern unsigned int nr_possible_cpus;
> +
>   #ifdef CONFIG_CPUMASK_OFFSTACK
>   /* Assuming NR_CPUS is huge, a runtime limit is more efficient.  Also,
>    * not all bits may be allocated. */
> @@ -81,8 +84,10 @@ extern const struct cpumask *const cpu_present_mask;
>   extern const struct cpumask *const cpu_active_mask;
>
>   #if NR_CPUS>  1
> -#define num_online_cpus()	cpumask_weight(cpu_online_mask)
> -#define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
> +
> +#define num_online_cpus()	(nr_online_cpus)
> +#define num_possible_cpus()	(nr_possible_cpus)
> +
>   #define num_present_cpus()	cpumask_weight(cpu_present_mask)
>   #define num_active_cpus()	cpumask_weight(cpu_active_mask)
>   #define cpu_online(cpu)		cpumask_test_cpu((cpu), cpu_online_mask)
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 2060c6e..d520d34 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -604,9 +604,13 @@ EXPORT_SYMBOL(cpu_all_bits);
>   #ifdef CONFIG_INIT_ALL_POSSIBLE
>   static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly
>   	= CPU_BITS_ALL;
> +unsigned int nr_possible_cpus __read_mostly = CONFIG_NR_CPUS;
>   #else
>   static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;
> +unsigned int nr_possible_cpus __read_mostly;
>   #endif
> +EXPORT_SYMBOL(nr_possible_cpus);
> +
>   const struct cpumask *const cpu_possible_mask = to_cpumask(cpu_possible_bits);
>   EXPORT_SYMBOL(cpu_possible_mask);
>
> @@ -614,6 +618,9 @@ static DECLARE_BITMAP(cpu_online_bits, CONFIG_NR_CPUS) __read_mostly;
>   const struct cpumask *const cpu_online_mask = to_cpumask(cpu_online_bits);
>   EXPORT_SYMBOL(cpu_online_mask);
>
> +unsigned int nr_online_cpus __read_mostly;
> +EXPORT_SYMBOL(nr_online_cpus);
> +
>   static DECLARE_BITMAP(cpu_present_bits, CONFIG_NR_CPUS) __read_mostly;
>   const struct cpumask *const cpu_present_mask = to_cpumask(cpu_present_bits);
>   EXPORT_SYMBOL(cpu_present_mask);
> @@ -628,6 +635,8 @@ void set_cpu_possible(unsigned int cpu, bool possible)
>   		cpumask_set_cpu(cpu, to_cpumask(cpu_possible_bits));
>   	else
>   		cpumask_clear_cpu(cpu, to_cpumask(cpu_possible_bits));
> +
> +	nr_possible_cpus = cpumask_weight(cpu_possible_mask);
>   }
>
>   void set_cpu_present(unsigned int cpu, bool present)
> @@ -644,6 +653,8 @@ void set_cpu_online(unsigned int cpu, bool online)
>   		cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
>   	else
>   		cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits));
> +
> +	nr_online_cpus = cpumask_weight(cpu_online_mask);
>   }
>
>   void set_cpu_active(unsigned int cpu, bool active)
> @@ -662,9 +673,11 @@ void init_cpu_present(const struct cpumask *src)
>   void init_cpu_possible(const struct cpumask *src)
>   {
>   	cpumask_copy(to_cpumask(cpu_possible_bits), src);
> +	nr_possible_cpus = cpumask_weight(cpu_possible_mask);
>   }
>
>   void init_cpu_online(const struct cpumask *src)
>   {
>   	cpumask_copy(to_cpumask(cpu_online_bits), src);
> +	nr_online_cpus = cpumask_weight(cpu_online_mask);
>   }

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